(1) Field of the Invention
This invention relates to novel cold rolling oil for steels and more particularly, to cold rolling oils for steels which comprise a lubricating oil component and a water-soluble cationic polymer compound or a water-soluble amphoteric polymer compound and which have good lubricity, surface cleanability with excellent heat resistance and oxidative stability.
(2) Description of the Prior Art
In recent years, there is a tendency toward the use of a mill-clean rolling oil in order to omit the cleaning step in cold rolling. The mill clean rolling oil should meet the following two characteristic requirements.
(i) Free of any contamination or stain on the sheet surface caused by carbon component in a rolling oil during annealing with a good surface appearance being obtained (hereinafter referred to as a resistance to stain by annealing or mill cleanability).
(ii) Having good lubricity during rolling without involving a galling phenomenon called heat streak or a vibration phenomenon called chattering (hereinafter referred to as lubricity).
In order to make good mill cleanability of (i), it is general that the amounts, in rolling oils, of aliphatic acids, fat and fatty oils and organic polymer compounds which are apt to produce residual carbon on annealing are reduced as small as possible, but rolling oils comprised mainly of volatile or decomposable components, i.e. mineral oils and synthetic esters, are in use.
However, such rolling oils have poor adsorptivity on steel materials and poor oil film formability whithin a roll contact arc, thus being substantially poor in lubricity. If the lubricity of (ii) is intended to be improved, large amounts of fat and fatty oils and aliphatic acids have to be used such as in beef tallow rolling oils, leading to a lowering of mill cleanability.
The mill clean rolling oil which is provided for the purpose of omitting the cleaning step must satisfy the two requirements which are contrary to each other. Accordingly, existing mill clean rolling oils are applied only to sheet gage steels whose finished thickness is relatively large and which are rolled under mild conditions (e.g. a finished thickness over 0.8 mm).
The present inventors made intensive studies to provide lubricating oils for cold rolling which overcome the drawback of the known mill clean rolling oils and which met the above requirements (i) and (ii). As a result, it was found that a specific type of composition comprised of a predetermined amount of monoesters of aliphatic carboxylic acids and aliphatic alcohols, a predetermined amount of an ester which is obtained by subjecting at least one of a dimer acid and/or polymerized acid of a higher fatty unsaturated acid to condensation with polyols under heating conditions and reacting remaining carboxylic acid groups or hydroxyl groups of the resulting polyester with an alcohol or aliphatic acid, had good mill cleanability and lubricity without involving oil staining. This composition was applied for patent (Japanese Laid-open Application No. 59-33395).
However, recent rapid developments in rolling equipments and techniques enable one to permit a high rolling speed and mass production. This leads to a severer requirement for rolling oil with regard to lubricity, circulation stability, workability, waste water treatability and the like. There is now a further demand for development of rolling oils which satisfy this requirement.
It will be noted that the above requirement could not be met by conventional rolling oils using emulsifiers because of various difficulties involved therein. More specifically, with known rolling oils using emulsifiers, the type and amount of emulsifier were changed to change amounts of a rolling oil and an oil deposited on a rolled steel (i.e. plateout amount), thereby controlling rolling lubricity. In the rolling oil using emulsifiers, however, the plateout amount and the liquid circulation stability tend to conflict to each other. In other words, if the stability of emulsion is increased, the amount of plateout on rolled steel decreases with insufficient lubricity. On the contrary, when the plateout amount is increased, the emulsion becomes unstable, so that various troubles are involved on use by circulation.